The details were given in a paper [1] at the ISSCC conference and it actually looked like the real thing, so this definitely caught the attention of our image-sensor experts inside TechInsights. Confirming its production status, at Mobile World Congress, Sony announced their Experia XZ Premium and XZs phones, with the Motion Eye camera system capable of 960 fps. We got hold of an Xperia XZs as soon as it was available, cross-sectioned the rear-facing camera chip, and lo and behold, we indeed have a three-stack; the CMOS image sensor (CIS) is mounted face-to-back on the DRAM, which is face-to-face with the image signal processor (ISP).

In its earlier 19 Mp image sensor, Sony used dual digital/analog converters to digitize the pixel data, and now we have a 4-tier construction to increase readout speed, and improve processing capability. The DRAM is used to temporarily store the high-speed data, and then output it at the optimal rate for the sensor interface. This design allows it to read a still image of 19.3 million pixels in only 1/120 of a second, and in movie mode it can go up to ~1000 fps, ~4x and ~8x (respectively) faster than the earlier product. This block diagram illustrates the signal path.

Given that the DRAM is sandwiched between the CIS and the ISP, the high-speed data has to go through the memory chip to the ISP, and then back-and-forth until it is output through the I/F (interface) block of the ISP, at a conventional speed suitable for the applications processor.

That’s a pretty compressed version of the sensor operation; the paper goes into more detail, including how the 960 fps imaging and slow-motion works. Sony’s schematic illustrates the three-stack.

According to the paper, the pixel array is in the top die, the DRAM array and row drivers are in the middle die, and the remaining blocks are in the bottom ISP die. We don’t have die photos of all three dies yet, but the paper had some images.

Sony also tells us that the CIS is made in a 90 nm, 1 Al, 5 Cu technology, the DRAM is a 1 Gb, 30 nm (3 Al, 1 W) part, and the ISP is a 40 nm, 1 Al, 6 Cu device. It was undoubtedly a challenge to get all the functionality required into three dies of the same size, without wasting silicon area.

Given that the DRAM die also has the CIS row drivers on it, then it must have been designed as a custom part, and is not one of the TSV-enabled (TSV = through-silicon via) commodity DRAMs that we have seen in recent years. Sony’s cross-section also shows that the center die has a thick back oxide and landing pad for TSVs coming down from the CIS above.

Interesting question! I'm not a DRAM expert, but refresh times are usually of the order of ms, so I guess conceptually the 960 fps mode could just pump the data straight in and out; anything slower would need refresh. However DRAM without refresh would change the design parameters hugely, so I would assume that it is used; just one or two cycles.